Field of the Invention
The present invention relates generally to a semiconductor laser with a rewritable wavelength stabilizer, and more particularly pertains to a semiconductor laser diode (LD) with a rewritable wavelength stabilizer, for example a grating, which makes it possible to change the wavelength of LDs after their fabrication.
The present invention applies to single-longitudinal mode LDs, such as DFB-LDs (distributed feedback laser diodes) or DBR-LDs (distributed Bragg reflector laser diodes), which are widely used in optical communication systems. The oscillation wavelengths of these LDs are determined by the grating period and by the structure (dimensions and materials) of their waveguide. These structural parameters must be decided before their fabrication, and also are not easily reproducible in the fabrication process, and therefore, it is normally impossible to change their wavelengths after finishing their fabrication.
Semiconductor laser diodes (LDs) have become very useful devices for optical communications. However, recent wavelength-division-multiplexing (WDM) systems require precise control of their wavelength. The oscillation wavelength of LDs is determined by their waveguide material and structure. Therefore, it is impossible to change their wavelength after finishing their fabrication. This makes it difficult to control the wavelength of LDs. If LDs with the required wavelengths cannot be obtained, then they must be refabricated from the first step of their fabrication process. The wavelength controllability of LDs is not sufficient for mass production of LDs suitable for WDM. Therefore, the production yield of LDs to obtain the required wavelength is quite low in mass production.
The present invention preferably uses materials which have a nonconventional local photorefractive effect which makes it possible to erase and rewrite gratings, although conventional nonlocal photorefractive materials may also be suitable for some embodiments. By using these photorefractive materials as reflective mirrors of the LD cavities, the operational wavelength of the LDs can be changed at any time by changing the grating period written into the photorefractive materials.
The grating can be erased by heating the material above an annealing temperature which depends upon the material. A grating can be rewritten after cooling by illuminating the material in order to change DX centers into a metastable state, thereby creating a diffraction grating. Therefore, the grating in these materials can be repeatedly erased and rewritten. This makes it possible to change the wavelength of the LDs at any time after their fabrication.
Accordingly, it is a primary object of the present invention to provide a semiconductor laser with a rewritable wavelength stabilizer.
The present invention solves the problem of wavelength control of LDs after fabrication by providing a method and structure which makes it possible to change the wavelength of LDs after their fabrication.
In accordance with the teachings herein, the present invention provides a semiconductor laser diode comprising a laser mirror made of a grating written in a photorefractive material, in which the oscillation wavelength of the laser diode is determined by the period of the grating. This allows the refractive index of the grating to be changed by illuminating the photorefractive material after cooling thereof to a temperature at which nearly half of the doped impurities form DX centers.
In greater detail, the grating can be erased by heating the photorefractive material to a temperature at which most DX centers are ionized, which erases the grating. Thereafter the photorefractive material is cooled again to a temperature at which nearly half of the impurities become DX centers, and a new grating can be written in the photorefractive material. The wavelength of the semiconductor laser can be changed repeatedly by erasing and rewriting the grating therein.
The photorefractive material is maintained and operated at a low temperature to maintain the grating semi-permanently therein while the temperature remains low. A thermoelectric Peltier cooler can be used to maintain the photorefractive material at a low operating temperature.
The grating can be written in the photorefractive material by a holographic lithography method using a laser to create optical interference on the photorefractive material between two laser beams divided from the laser.
The photorefractive material is preferably used as a reflective mirror of the cavity of the laser diode, and can be used as a distributed bragg reflector (DBR) mirror in the laser diode, or as a distributed feedback (DFB) mirror.
The photorefractive materials can be semiconductors with impurities which form DX centers, such as Ga doped CdF2, or Si or Te doped AlGaAs.
The laser diode can be optically coupled with the photorefractive grating mirror by a lens by collimating an optical beam from a facet of the laser diode and directing it into the photorefractive mirror. An antireflective coating can be placed on the laser diode facet facing the lens to prevent FP-mode laser oscillations.
The laser diode can be fabricated by first preparing a conventional FP laser diode with buried heterostructure, etching a portion of the laser diode cavity from the surface to the depth of the substrate, and then embedding the removed portion with the photorefractive material.
An index matching layer can be introduced between the laser diode waveguide and the photorefractive mirror to minimize the effect of a large refractive index difference between the laser diode waveguide material and the photorefractive material.